The Full Form of BMU is Building Maintenance Units.

A Building Maintenance Unit (BMU) is an automatic, remote-controlled, or mechanical device, usually suspended from the roof, which moves systematically over some surface of a structure while carrying window washers or mechanical robots to maintain or clean the covered surfaces.

Building Maintenance Units are required on the more complicated building designs, and provide a sophisticated façade access solution. They are usually mounted permanently at roof level, and provide full lateral, horizontal and vertical movement of the working platform, in order to access all surfaces of a building.
The system is designed for scheduled façade maintenance e.g. window washing & external cleaning, and unscheduled maintenance e.g. replacement of glass/façade panels and other repairs.